A super-Earth and a sub-Neptune orbiting the bright, quiet M3 dwarf TOI-1266

We report the discovery and characterisation of a super-Earth and a sub-Neptune transiting the bright (\(K=8.8\)), quiet, and nearby (37 pc) M3V dwarf TOI-1266. We validate the planetary nature of TOI-1266 b and c using four sectors of TESS photometry and data from the newly-commissioned 1-m SAINT-EX telescope located in San Pedro Mártir (Mexico). We also include additional ground-based follow-up photometry as well as high-resolution spectroscopy and high-angular imaging observations. The inner, larger planet has a radius of \(R=2.37_{-0.12}^{+0.16}\) R\(_{\oplus}\) and an orbital period of 10.9 days. The outer, smaller planet has a radius of \(R=1.56_{-0.13}^{+0.15}\) R\(_{\oplus}\) on an 18.8-day orbit. The data are found to be consistent with circular, co-planar and stable orbits that are weakly influenced by the 2:1 mean motion resonance. Our TTV analysis of the combined dataset enables model-independent constraints on the masses and eccentricities of the planets. We find planetary masses of \(M_\mathrm{p}\) = \(13.5_{-9.0}^{+11.0}\) \(\mathrm{M_{\oplus}}\) (\(